Each threat detected added may have x number of variants as part of that detection thus making the total number of detections higher than it looks. Notice in most cases Symantec give the name of the detection without a sub category, like Trojan.Linkoptimizer. No Trojan.Linkoptimizer.a, .b etc. although I have to say that theory doesn't always follow as there are some listings with sub categorisation, such as Backdoor.Lassrv.B.